A wide variety of metallization approaches for silicon integrated circuits have been used in the past but in current commercial practice a relatively few highly developed and proven systems are considered standard. Nearly all metallization systems use aluminum and aluminum alloys as the predominant interconnect material, but it has been known for some time that aluminum does not form a reliable contact to silicon for substrate connections, or to polysilicon at the gate level interconnect.
An approach to overcome this that has gained wide acceptance is to interpose a barrier layer between the aluminum alloy and the silicon or polysilicon surface. The barrier layer is typically Ti/TiN formed using a cluster tool by sputtering titanium from a titanium target in argon, then introducing nitrogen into the sputtering chamber to produce an overcoating of TiN. Alternatively, the TiN is deposited in a separate chamber of the cluster tool. The titanium layer forms a stable interface and promotes adhesion with silicon, and the TiN layer acts as a barrier between the silicon and the aluminum alloy plug. However, with ever diminishing lithographic design rules the aspect ratio of the windows and the vias shrinks to the point where bottom and sidewall coverage of the Ti/TiN barrier is inadequate. This creates several potential problems. One of the most serious of these is the formation of aluminum spiking in regions where the layers are incomplete or too thin.
To address bottom coverage attempts have been made with collimation techniques for the sputtered beam to fill the bottom of the window or via. While this improves bottom coverage, sidewall coverage is still a problem due to the anisotropic nature of the deposition.
Another approach to non-uniform barrier layer coverage has been to deposit the TiN barrier layer using CVD. See e.g., Sherman, "Titanium Nitride Deposition in a Cold Wall CVD Reactor", Materials Research Society--Proceedings of the 1988 Workshop, pp. 323-329 (1989); Pintchovski et al "LPCVD Titaniun Nitride--Deposition, Properties, and Applications To ULSI", Materials Research Society--Proceedings of the 1988 Workshop, pp. 275-282 (1989); This deposition technique produces conformal coatings, improving both the step coverage (sidewall) and the bottom coverage. Since the quality and coverage of CVD deposited films is superior to sputtered films the thickness of the TiN barrier layer can be substantially reduced, from typically a thousand angstroms for sputtered barriers to 20-400 angstroms for CVD barrier layers.
TiN is typically deposited by CVD using two precursor materials, TiCl.sub.4 or an organic titanium compound, e.g. tetrakis--(dimethylamino) titanium (TDMAT) or tetrakis--(diethylamino) titanium TDEAT. For further details of this process see e.g. Konecni et al, "A Stable Plasma Treated CVD Titanium Nitride Film For Barrier/Glue Layer Applications", 1996 VMIC Conference 1996 ISMIC--106/96/0181(c), pp. 181-183, 1996.
CVD TiN deposited using TiCl.sub.4 has a very high resistivity unless it is deposited at elevated temperatures, typically 500-550.degree. C. Therefore its application may be limited to contact (window) plugs. Moreover, TiN produced using TiCl.sub.4 has a high Cl impurity content. Chlorine is corrosive and at high levels is unacceptable in IC environments. The organic precursors, e.g. TDMAT, are gaining popularity in the field because of the highly conformal nature of the coating, low deposition temperature, and simple processing techniques. However, we have discovered that this technique has several limitations.
CVD-TiN deposited for barrier layers requires a post deposition treatment in N.sub.2 to stabilize the layer (see Konecni et al ref. above). Usually, the treatment is carried out using a plasma in the same chamber. The organic precursors TDMAT and TDEAT leave residual carbon and oxygen. The stabilization treatment assists in removing carbon and replaces the amine groups in the TiN film. However, the film typically undergoes crystallization, with substantial shrinkage e.g. 50%, particularly in the field areas and at the bottom of the window/via where plasma exposure is highest. The sidewalls receive less exposure to the plasma and therefore retain most of the original thickness. In fact, sidewalls show greater than 100% (approx. 110-130%) of the field TiN thickness after plasma treatment. This difference in exposure results in a difference in the morphology of the film between the sidewalls and the bottom. The sidewalls show some crystallization but remain mostly amorphous. The bottom regions, however, have a highly crystalline or nano-crystalline morphology. The sidewall shows a thin layer of crystalline material on an amorphous underlayer while the field and bottom regions show crystalline or nano-crystalline features. Since the degree of transformation varies in a plasma-treated film, depending upon the location in a feature such as a window or via, the crystallographic texture of the treated TiN film will also vary. When Al is deposited on the plasma treated TiN films, because of the varying texture, the Al texture will also vary. Consequently, for illustration, the Al will have a &lt;111&gt; texture at the bottom and the field regions, and non-&lt;111&gt; texture on the sidewalls. The amount of Al &lt;111&gt; texture will also depend on the degree of TiN transformation, plasma treatment parameters, processing parameters, etc.
Since the Al is of varying texture along various regions of the opening, the reliability of the Al plug and film will be suspect, since Al films with &lt;111&gt; texture are reported to have good resistance to electromigration and stress-induced voiding. Therefore, films of varying texture will have questionable reliability.
Moreover, since the plasma exposure at different locations of the opening (contact or via) varies, the grain size of the plasma-treated TiN (hereafter called transformation region) will also vary. The non-uniform variation in grain size, attendant with very small grains and therefore a large number of available diffusion paths per unit area, will allow localized migration of Al and an increased propensity for junction leakage and spiking.